1. Field of the Invention
The present invention relates an apparatus for writing images by injecting ink and, particularly, to an electrostatic hydrodynamic jet writing method using electro-rheological fluid and apparatus therefor which uses electro-rheological fluid and controls the ejection of the fluid by using one electrical potential for varying the viscosity of the fluid and another electrical potential for electrostatic induction of the fluid.
2. Description of Prior Art
Electro-rheological fluid is well-known for its electro-field responsiveness. The electro-rheological fluid was first disclosed in U.S. Pat. No. 2,417,850 by Winslow in 1943, and has thereafter been proposed in various forms as disclosed in U.S. Pat. No. 3,047,507 by Winslow, USSR patent 1391951 by Lysenkov and U.S. Pat. No. 4,812,251 by Stangroom.
The electro-rheological fluids proposed by the aforementioned publications are basically made of electric-viscosity liquid containing a powdery additive of a minute particle diameter. Thus, if an electric field is applied to the fluid, the viscosity thereof varies. The viscosity of these electro-rheological fluids has been known to vary in proportion to the strength of the applied electric field. This is referred to as electric viscosity effect. The electric viscosity effect is a phenomenon whereby the viscosity of a fluid is varied depending on the strength of an applied electric field, and varies almost concurrently with the electric field application. Among the above-described electro-rheological fluids, there is one whose viscosity varies from a liquid state to a nearly solid state by application of an electric field even below 10 KV/mm.
Using electro-rheological ink having such an electric viscosity effect and an appropriate controller, an intended image can be written. An apparatus for writing images using such electro-rheological ink is disclosed in Japanese laid-open patent publication Sho 55-117663.
FIG. 1 illustrates a conventional head for injecting such electro-rheological ink comprising a nozzle sheet 1 and a pair of support sheets 2 and 3 stacked about nozzle sheet 1. Nozzle sheet 1 has an ink reservoir la for receiving ink in a certain quantity and a nozzle 1b for ejecting ink therefrom. Upper support sheet 2 has an ink supplying aperture 2a and an electrode plate 4. Lower support sheet 3 has another electrode plate 4'. In this configuration, a predetermined pressure is kept with respect to the inside and outside of nozzle 1b. The viscosity of the ink inside the nozzle is varied by a writing potential 5 applied to the pair of electrode plates 4 and 4'. When the ink has a low viscosity, i.e. liquid, the ink is ejected due to the pressure difference between the inside and outside of the nozzle. That is, the viscosity of the ink inside the nozzle is varied by the strength of the electric field formed by the two electrodes. Thus, the ink is not ejected when it has a high viscosity, i.e. nearly solid, and is ejected when it has a low viscosity. Such an apparatus for ejecting electro-rheological ink, according to the writing potential, requires an additional means for creating the pressure difference inside and outside the nozzle and holding the pressure difference. This causes the apparatus to be more complicated and expensive while impeding its miniaturization.
An electrostatic hydrodynamic jet method has been proposed and disclosed in U.S. Pat. No. 3,060,429 to solve the aforementioned disadvantage in apparatuses using the electro-rheological fluid. FIG. 2 illustrates such an electrostatic hydrodynamic apparatus which utilizes an accelerating potential 9 applied, according to a writing signal, between a nozzle 6a at the end of an ink reservoir 6 and a platen 8 onto which paper 7 is guided. In this device, the ink is ejected from the nozzle onto the paper due to the static electricity induced into the ink by the accelerating potential difference (the potential difference between the nozzle and the platen). This apparatus is simplified, in comparison with the above-described electro-rheological fluid writing apparatus, because it does not produce pressure and, in turn, the pressure does not need to be controlled. However, even though continued research has greatly reduced the necessary writing potential, it is still almost the same as the accelerating potential. Consequently, a high voltage must still be controlled, which is difficult, resulting in a more complicated and undesirable method and apparatus for high-resolution writing.